Ohmic contact configuration

ABSTRACT

A contact configuration has an ohmic contact between a metalization layer and a semiconductor body of monocrystalline semiconductor material. An amorphous semiconductor layer is formed between the metalization layer and the monocrystalline semiconductor body. The layer is formed of the same semiconductor material as the body. The contact configuration is either produced by applying amorphous semiconductor material on the semiconductor body (e.g., sputtering, vapor deposition, glow discharge) or by damage formation in the semiconductor body.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention lies in the semiconductor technology field. Morespecifically, the invention relates to a contact configuration with anohmic contact between a metalization layer and a semiconductor body madeof a monocrystalline semiconductor material.

In order to produce an ohmic contact between a metalization layer and asemiconductor body, a sufficiently high doping concentration is requiredin the semiconductor material of the semiconductor body. By way ofexample, if the metalization layer comprises aluminum and thesemiconductor body comprises p-doped silicon, then the dopingconcentration in the surface region of the semiconductor body withrespect to the metalization layer should be at least 10¹⁷ doping atomscm⁻³. If the silicon of the semiconductor body is n-doped, then asurface doping concentration of even in excess of 10¹⁹ dopant atoms cm⁻³is required.

These minimum doping concentrations pose a problem if the regions of thesemiconductor body that adjoin the contact region with respect to themetalization layer are not intended to have a high emitter efficiency.This is because the injection behavior of an emitter depends cruciallyon the dopant dose introduced into its region. In a semiconductor body,however, even with a deposition very near the surface for example by wayof ion implantation and a subsequent annealing step, high surface dopingconcentrations cannot be produced with arbitrarily small dopant doses ofthe order of magnitude of less than 10¹³ dopant atoms cm⁻², for example,without significant redistribution.

At the present time, the contact to the body zone in power MOSFETs andIGBTs (insulated gate bipolar transistor) is preferably connected to thesource electrode by a heavily doped p-conducting region with the lowestpossible resistance in order that the pn junction between body zone andsource zone is not forward-biased with respect to the source zone in theevent of a high shunt current. For this would lead to a so-called“latch-up” of the power MOSFET or IGBT, which prevents controllabilityvia the gate and brings about destruction of the power MOSFET or IGBT inthe absence of external additional measures.

The body diode thus integrated between body zone and source zone has theeffect, in a power MOSFET or IGBT, that the latter is very heavilyflooded with charge carriers in the reverse direction. The commutationproperties of the body diode are very poor due to the high p-type dopingof the body zone on the front side of the power MOSFET. By contrast,IGBTs exhibit a certain reverse blocking capability due to the rear-sidepn junction with respect to the collector zone. Here, the IGBT isalready flooded with charge carriers in normal forward operation, whichcharge carriers then have to be depleted in the event of transition tothe blocking state. The resultant charge carrier current to the cellthen has to be conducted away with a sufficiently low resistance via thep-conducting body zone to the body contact.

It is quite generally the case with bipolar transistors and diodes thata relatively high doping of the regions in the vicinity of themetalization layer with which contact is to be made leads to an oftenundesirably strong emitter, which results in a correspondingly highdegree of flooding of the component with charge carriers and thus inhigher switching losses.

In the prior art, the use of power MOSFETs in group circuits inparticular at relatively high voltages of above about 300 V has beenpossible only to a very limited extent. At relatively low voltages ofbelow 300 V, the switching losses in such power MOSFETs are relativelyhigh. At the present time, the required latch-up strength of powerMOSFETs or IGBTs is ensured through precise design of their cells andcomplicated fabrication methods.

In the case of bipolar transistors and diodes, low emitter efficienciescan be ensured on the one hand through correspondingly low dopant dosesand on the other hand through optimized doping methods. Reference ishad, in this context, to the commonly assigned published applicationU.S. Ser. No. 2003/0122151 A1 and German published patent application DE100 31 461 A1, for example, which describe a high-voltage diode whereinthe doping concentrations of an anode region and of a cathode region areoptimized with regard to the basic functions of “static blocking” and“forward state.” However, all of these measures are usually insufficientfor achieving a weak emitter that is desired in many cases.

For this reason, it is necessary to employ additional methods by which asubsequent weakening of the emitter efficiency is achieved by means oflocal or homogeneous setting of the charge carrier lifetime. What isparticularly of significance here is a lowering of the charge carrierlifetime through local damage of the semiconductor crystal lattice in orin the vicinity of the emitter by irradiation with high-energy particlessuch as, for example, electrons, protons or helium atoms. What isdisadvantageous about such a procedure, however, is once again thesusceptibility of completed components to process variations.

SUMMARY OF THE INVENTION

It is accordingly an object of the invention to provide an ohmic contactconfiguration, which overcomes the above-mentioned disadvantages of theheretofore-known devices and methods of this general type and specifiesa configuration with an ohmic contact between a metalization layer and asemiconductor body that can be produced in a simple manner and is ableto ensure a low emitter efficiency. Moreover, the intention is toprovide an advantageous method for producing such a contactconfiguration.

With the foregoing and other objects in view there is provided, inaccordance with the invention, a contact configuration, comprising:

a semiconductor body of semiconductor material in a monocrystallinephase;

a metalization layer; and

a layer of said semiconductor material in a substantially amorphousphase disposed between said semiconductor body and said metalizationlayer, for forming an ohmic contact between said metalization layer andsaid semiconductor body.

In other words, a contact configuration of the type mentioned in theintroduction is provided with a layer made of the amorphoussemiconductor material of the semiconductor body, the layer beingprovided between the semiconductor body and the metalization layer.

The invention is thus based on the completely novel insight concerningthe usability of amorphous silicon: previously, amorphous silicon hasbeen used for anti-reflection layers and for passivation. The inventionnow envisages that amorphous silicon may serve, this being completelynovel, as a contact material between a metalization layer and asemiconductor body comprising silicon.

However, the invention is not restricted to silicon: rather, it cangenerally also be applied to other semiconductor materials, such as, forexample, to silicon carbide, compound semiconductors etc. Thus, by wayof example, an amorphous silicon carbide layer can effect an ohmiccontact between a metalization layer and a silicon carbide semiconductorbody.

The contact configuration according to the invention thus enables anohmic junction between, in particular, a lightly doped siliconsemiconductor body and a metalization layer applied thereto by anintermediate layer made of amorphous silicon being deposited onto thesilicon of the semiconductor body. On account of its high defectdensity, amorphous silicon has the desired property of forming an ohmiccontact between the amorphous silicon layer, on the one hand, and themetalization layer deposited thereon, on the other, as well as betweenthe amorphous silicon layer on the one hand, and the crystalline siliconof the semiconductor body on the other hand. This specifically holdstrue even when the preferably n-conducting doping in the amorphoussilicon layer is present only in a low concentration.

Amorphous silicon vapor-deposited or sputtered onto a siliconsemiconductor body is generally n-conducting after a heat treatmentwhich follows its deposition, which heat treatment may preferablyproceed at about 350° C. to 450° C. In this case, the amorphous siliconmay already contain a relatively high concentration of hydrogen,depending on its production process. Since the sheet resistance thatresults in an amorphous silicon layer is relatively high, it may beexpedient for hydrogen atoms additionally to be incorporated into theamorphous silicon layer in order to increase the n-type doping in atargeted manner.

The incorporation of the hydrogen atoms into the amorphous silicon layermay be effected for example by the heat treatment which follows thedeposition, and which is carried at about 350° C. to 450° C., beingperformed in a hydrogen-containing atmosphere. A further possibilityconsists in producing the amorphous silicon layer by means of a glowdischarge process in a silane atmosphere (SiH₄ atmosphere) or else incarrying out the sputtering process itself in a hydrogen-containingatmosphere.

A primary advantage of the contact configuration according to theinvention is that it enables an ohmic contact on an n-doped or elsep-doped semiconductor body, and in particular on a silicon semiconductorbody, without the need for the contact to have a high emitterefficiency, since the emitter efficiency remains low due to theamorphous structure of the deposited layer.

In addition to or instead of the doping by means of hydrogen, it is alsopossible to provide an amorphous silicon layer with other n-dopingsubstances, such as phosphorus, for example. Such an additional dopingis preferably performed since hydrogen-doped silicon can be electricallyactively doped more easily with phosphorus, for example, or else—for thecase of a targeted p-doping—with boron, for example.

The contact configuration according to the invention is advantageouslyemployed for example for the source contact of a MOS component, that isto say of a MOSFET or IGBT, for example. In the case of such a MOSFETcomponent, it is possible to dispense with a short circuit between bodyzone and source zone owing to the poor emitter efficiency. In this case,n-doped amorphous silicon can be deposited either directly on a p-dopedsemiconductor body (bulk) as source zone or channel terminal or on amore weakly n-doped source zone as contact material.

If such an n-doped emitter is employed for diode structures, then thepossibility of producing defects in the depth of the siliconsemiconductor body through an additional irradiation by means of protonsor helium atoms is also afforded, which defects may be provided withhydrogen during the above-described heat treatment process and then formdonors. This process can thus be used to form an upstream field stopzone which is desired for many diode structures and, inter alia, leadsto a softer switch-off (this will be discussed in more detail furtherbelow in connection with FIG. 1). This zone may also lead to a targetedraising of the emitter efficiency. However, it is also possible toprovide a raising of the emitter efficiency of that semiconductor regionof the semiconductor body which is coated with amorphous silicon throughan additional moderate conventional doping of the crystalline siliconregion situated in direct proximity to the amorphous silicon layer, forexample by means of phosphorus atoms in the case of an n-type doping andfor example by means of boron atoms in the case of a p-type doping.

In principle, it is also possible to use the contact configurationaccording to the invention to produce a stable ohmic contact on alightly doped p-conducting region for an IGBT, for example. This contactis likewise distinguished by a low emitter efficiency. In this case, theamorphous silicon may also be produced in p-conducting fashion throughsuitable doping. In this case, too, the emitter efficiency may easily beraised as required through a moderate additional doping of the region ofthe crystalline silicon which is situated in the region of the interfacewith the amorphous silicon layer. Thus, in IGBTs for relatively highswitching frequencies, at the present time preferably weak p-conductingemitters are used for reducing the switch-off losses. The use ofamorphous silicon as contact material in this case makes it possible tofurther reduce the p-type dose and thus the switching losses. At thepresent time, the minimum emitter efficiency is limited here by theohmic contact-connectability.

The contact configuration according to the invention makes it possibleto protect specific regions in components which become very hot onaccount of instances of current splitting by the efficiency of an n- orp-conducting emitter formed by an amorphous silicon layer being locallyattenuated in the critical component regions. Such an amorphous siliconlayer can be produced in a self-aligning manner by exploiting the effectthat amorphous silicon starts to recrystallize at temperatures in therange above 600° C., which increases the contact resistance. Thus, ifthe component is operated above a noncritical current density range overa certain period of time, then the injection can also be locallyattenuated on account of the local temperature increase and the locallyincreased contact resistance resulting therefrom. This reduces theinjection of such an emitter in the critical component regions, that isto say for example in the edge region of diodes during dynamic operationor, in pressure contact IGBTs, in the region situated below the edge ofthe pressure piece. This leads to a load relief during switch-off in theedge region in the case of the diodes, for example.

In an advantageous manner, in the case of a contact configuration with ahydrogen-containing and generally additionally doped silicon layer, anoutdiffusion of hydrogen atoms, which already occurs to an appreciableextent at temperatures in the region of 400° C., already suffices toimpair the injection behavior of the emitter in locally targetedfashion. An alternative without utilizing this effect is locallyreducing the emitter efficiency by locally driving out the doping fromthe amorphous silicon by means of a locally delimited input of heat fromoutside. Such an input of heat may be effected for example by means of aheated grid or through radiation which acts locally, such as laserradiation for example, or is locally shielded, which can be done forexample by means of a screen in an RTA furnace (RTA=Rapid ThermalAnnealing). It is also possible to allow the radiation to act in pulsedfashion.

The contact configuration according to the invention may be produced bydeposition by means of vapor deposition or sputtering of amorphoussemiconductor material, such as, in particular, silicon or siliconcarbide. However, it is also possible not to deposit the amorphoussemiconductor material but rather to amorphize the surface ofmonocrystalline semiconductor material. With silicon, then, no amorphoussilicon is deposited in this case. Rather, monocrystalline silicon issubjected to a damage process in order to amorphize it in regions wherethe intention is to create a contact configuration with an ohmiccontact.

It is advantageous, particularly for contact configurations on the rearside of a semiconductor wafer, to produce a damage by means of animplantation with a non-doping element. With this procedure, the actualemitter is then implanted particularly shallowly, so that practicallyall of the implanted atoms remain in the region of the damage. As analternative, there may be a low dose of this emitter in the crystallineregion as well, wherein case this dose should be so low that thetargeted weak emitter efficiency is not exceeded. Elements of the thirdperiod of the periodic table, such as silicon or argon, for example, arepreferably suitable for a damage implantation. These elements have arelatively low amorphization dose in the region of 5·10¹⁴ cm⁻² and, onthe other hand, have significantly greater penetration depths thanelements of the fourth period of the periodic table.

Other features which are considered as characteristic for the inventionare set forth in the appended claims.

Although the invention is illustrated and described herein as embodiedin an ohmic contact configuration, it is nevertheless not intended to belimited to the details shown, since various modifications and structuralchanges may be made therein without departing from the spirit of theinvention and within the scope and range of equivalents of the claims.

The construction and method of operation of the invention, however,together with additional objects and advantages thereof will be bestunderstood from the following description of specific embodiments whenread in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph plotting a fundamental doping profile of a 1200 Vdiode, with the doping plotted as a function of an anode distance;

FIG. 2 is a diagrammatic sectional illustration through a contactconfiguration according to the invention;

FIG. 3 is a sectional view of a trench component with the contactconfiguration according to the invention; and

FIG. 4 is a sectional view of a planar component with the contactconfiguration according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the figures of the drawing in detail and first,particularly, to FIG. 1 thereof, there is shown a fundamental dopingprofile of a 1200 V diode with the contact configuration according tothe invention. In this case, the basic doping BD in charge carriers/cm³is plotted as a function of a distance d from the anode of the diode inμm. In the case of this diode, a rear-side contact comprises amorphoussilicon (a-Si) and has a basic doping of between 10¹³ and 10¹⁴ chargecarriers cm⁻³. The doping profile firstly exhibits a region with ahomogeneous basic doping, followed by a field stop zone with a highdoping. This field stop zone then undergoes transition to a layer madeof amorphous silicon on the rear side of the diode.

The field stop zone may be produced for example through an additionalirradiation by means of protons or helium atoms. The protons or heliumatoms produce defects in the depth of the semiconductor body, whichdefects are provided with hydrogen during a heat treatment process afterthe deposition of the amorphous silicon layer and form donors. Whereasthe hydrogen is already present in the semiconductor body in the case ofproton implantation, in the case of prior helium implantation it firsthas to be indiffused for example from the vapor phase or a plasma. Thedonors increase the doping in the region of the field stop zone abovethe homogeneous basic doping. The field stop zone has the advantage thatit ensures, inter alia, a softer switch-off of the diode.

FIG. 2 shows a diagrammatic sectional illustration through the contactconfiguration according to the invention. An amorphous semiconductorlayer 2 is disposed on a semiconductor body 1 made of monocrystallinesilicon or monocrystalline silicon carbide, for example. The amorphoussemiconductor layer 2 is likewise made of silicon or silicon carbide.The layer thickness of the layer 2 is in the nm range and may, forexample, lie between 2 nm and 100 nm or a few 100 nm. The dopingconcentration in the layer 2 is relatively low and, for example, liesbetween 10¹⁵ and 10¹⁶ charge carriers cm⁻³.

A metalization layer 3 is applied as contact on the layer 2. By way ofexample, aluminum or chromium or aluminum/chromium may be used for themetalization layer 3.

FIG. 3 shows, as a concrete exemplary embodiment of the contactconfiguration according to the invention, a sectional illustrationthrough a vertical trench MOSFET with an n-doped silicon semiconductorbody 1 into which are introduced trenches 4 filled with polycrystallinesilicon as gate electrode. A p-doped body zone 5 is situated in thesemiconductor body 1, at the top side thereof, an n-doped source zone 6being provided in turn at the top side of the body zone. The source zone6 and the body zone 5 are contact-connected by a metalization layer 3made of aluminum.

An n⁺-doped terminal zone 7 is additionally provided on the rear side ofthe semiconductor body 1. A drain contact 8 (D) is provided on theterminal zone.

According to the invention, the body zone 5 and the source zone 6, onthe one hand, and/or the n⁺-conducting contact zone 7 are now providedwith a p-doped or n-doped layer 2 made of amorphous silicon. The layer 2may be produced by vapor deposition, as has been explained above, orelse by amorphization.

FIG. 4 shows, as a further exemplary embodiment of the contactconfiguration according to the invention, a sectional illustrationthrough a planar IGBT with an n⁻-conducting silicon semiconductor body1, an additional lightly doped p-conducting collector layer 9, acollector contact layer 10 (K), p-conducting body zones 5, lightly dopedn-conducting source zone 6, gate electrodes 11 in an insulating layer 13made of silicon dioxide with a gate oxide 12 and an aluminummetalization layer 3.

Generally, the layer 9 may act as an emitter and be doped so weaklythat, without the amorphous layer 2, a Schottky contact or an ohmiccontact with a relatively high contact resistance would be produced.

According to the invention, layers 2 made of amorphous doped silicon areprovided below the aluminum metalization 3 in the body zone 5 and thesource zone 6 and/or between the p-conducting collector layer 9 and thecollector contact layer 10 made likewise of aluminum, in order to enablea relatively low or even negligible doping of the zone 6 and/or of thelayer 9. The layers 2 may be n-doped in the region of the source zone 6and body zone 5 and be p-doped in the region of the collector layer 9.It goes without saying that respectively opposite conduction types arepossible for the doping in this case as well.

The layers 2 may be produced by deposition by means of vapor depositionor sputtering in an optionally hydrogen-containing atmosphere, which maybe followed by a heat treatment at about 350° C. to 450° C. in likewisea hydrogen-containing atmosphere. However, it is also possible toproduce the amorphous layer 2 by means of a glow discharge process in anSiH₄ atmosphere. Finally, the amorphous layer need not actually bedeposited: rather, it is possible to amorphize the surface of thesemiconductor body 1 itself (cf. FIG. 2) by introducing a damage bymeans of implantation with a non-doping element, such as, in particular,an element of the third period of the periodic table, that is to saysilicon or argon, for example. This implantation may be effected with adose of about 5·10¹⁴ to 1·10¹⁶ cm⁻².

The layer 2 may, preferably, also be locally recrystallized in componentregions. This recrystallization may be performed at temperatures inexcess of about 600° C. Regions which are suitable for arecrystallization are those regions wherein the emitter efficiency isintended to be reduced compared with the rest of the emitter area.

1. A contact configuration, comprising: a semiconductor body ofsemiconductor material in a monocrystalline phase, said semiconductorbody having one of a trench component and a planar component formedtherein, said component being selected from the group consisting of adiode, a bipolar transistor, a MOSFET, and an IGBT; a metalization layerformed of a metal selected from the group consisting of aluminum,chromium, and aluminum/chromium; and a layer of said semiconductormaterial in a substantially amorphous phase disposed between saidsemiconductor body and said metalization layer, for forming an ohmiccontact between said metalization layer and said semiconductor body;said semiconductor material being silicon and said layer being a layerof amorphous silicon doped with hydrogen.
 2. The contact configurationaccording to claim 1, wherein said silicon semiconductor body isn-conducting in a region of said layer of amorphous silicon.
 3. Thecontact configuration according to claim 1, wherein said siliconsemiconductor body is p-conducting in a region of said layer ofamorphous silicon.
 4. The contact configuration according to claim 1,wherein said layer of amorphous semiconductor material has a thicknessin the order of magnitude of nanometers.
 5. The contact configurationaccording to claim 4, wherein said thickness of said layer lies between2 and 100 nm.
 6. The contact configuration according to claim 1, whereinsaid layer of amorphous semiconductor material has a doping of between10¹⁵ and 10¹⁶ charge carriers per cm³.
 7. The contact configurationaccording to claim 1, which comprises a field stop zone in saidsemiconductor body, said field stop zone adjoining said layer of saidamorphous semiconductor material.
 8. The contact configuration accordingto claim 1, which further comprises an additional layer in saidsemiconductor body in a region of said layer of amorphous semiconductormaterial, said additional layer forming an emitter.
 9. The contactconfiguration according to claim 8, wherein said additional layer andsaid semiconductor body are of a common conductivity type.
 10. Thecontact configuration according to claim 8, wherein said additionallayer and said semiconductor body having mutually opposite conductivitytypes.
 11. The contact configuration according to claim 8, wherein saidadditional layer is doped so weakly that, without said layer ofamorphous semiconductor material, said additional layer forms one of aSchottky contact or an ohmic contact with a relatively high contactresistance.
 12. The contact configuration according to claim 1, whereinsaid layer of amorphous semiconductor material is formed on at least oneof a front side and a rear side of said semiconductor body.
 13. Thecontact configuration according to claim 12, wherein said layer ofamorphous semiconductor material is formed to locally attenuate aninjection of charge carriers in critical component regions.
 14. Thecontact configuration according to claim 1, wherein said layer ofamorphous semiconductor material is locally recrystallized.
 15. Thecontact configuration according to claim 1, wherein in said amorphoussemiconductor material is silicon carbide.